Introduction
Fiber optic communication systems are becoming increasingly popular for data transmission due to their high speed and high data capacity capabilities. A number of very basic optical functions are required to permit the efficient transfer of large amounts of data over such systems and to maintain the operation of the system. Among these basic functions are those of wavelength division multiplexing and demultiplexing. Wavelength division multiplexing permits simultaneous transmission of a composite optical signal comprising multiple information-carrying signals, each such signal comprising light of a specific restricted wavelength range, along a single optical fiber. A multiplexer combines optical signals of different wavelengths from different paths onto a single combined path; a de-multiplexer separates combined wavelengths input from a single path onto multiple respective paths. Such wavelength combination and separation must occur to allow for the exchange of signals between loops within optical communications networks and to ultimately route each signal from its source to its ultimate destination.
Another basic function needed by fiber optic communication systems is that of independent control of the power levels of all signals comprising a wavelength division multiplexed optical transmission. Because of the power level expectations of receiver equipment within a fiber optic communication systems, all channels must be of a uniform power level. No channel can be significantly more intense than others. However, because of general non-uniform amplification by optical amplifiers and different routes traced by the various channels, re-balancing the channel powers is frequently required at various points.
Further, the exact form of the gain spectrum of the commonly utilized Erbium-Doped Fiber Amplifier (EDFA) type of optical amplifier can vary depending upon the amount of total optical power that is input to an EDFA. Because of such changing gain characteristics, the difference in amplification between channels may not be constant. Therefore, variable optical attenuator (VOA) apparatuses are generally utilized within optical communications networks so as to balance the powers carried by the various channels and to control the overall optical power of all channels.
FIG. 4 presents a known OADM architecture. A composite optical signal entering OADM 400 from an input fiber optic line is de-multiplexed into its component channels λ1, λ2, . . . , λn by de-multiplexer 402a. Simultaneously, a set of channels to be added are input to OADM 400 from add line 404 and de-multiplexed into their component channels by de-multiplexer 402c. The channels λ1 and λ′1 (if present) are directed to the 2×2 switch 406.1; the channels λ2 and λ′2 (if present) are directed to the 2×2 switch 406.2; and so on. In the example shown in FIG. 4, it is assumed that the add channels comprise only the two channels λ′1 and λ′2. Since each add operation is always paired with a concurrent drop operation, this implies that the channels λ1 and λ2 are dropped. Each of the 2×2 switches 406.1–406.n can be in either one of two states—a “cross state” or a “bar” state. In the example shown in FIG. 4, since the channels λ′1 and λ′2 are added, the two switches 406.1–406.2, which receive these channels, are in their “cross” states. Since no other channels are added, the switches 406.n (and all other switches) are in their “bar” states. Thus, the channels λ1 and λ′1 and λ′2 and λ′2 are switched such that the channels λ1 and λ2 are “dropped” to the multiplexer 402d whilst the channels λ′1 and λ′2 are directed to the multiplexer 402b. The non-dropped or “express” channels λ3–λn are all directed to the multiplexer 402b. The multiplexer 402b multiplexes the “added” channels λ′1 and λ′2 together with the “express” channels λ3–λn so as to be output as a single composite optical signal along the output fiber optic line. The multiplexer 402d multiplexes the two channels λ′1 and λ′2 so as to be output as a composite optical signal along the drop line 408.
Although the conventional OADM 400 performs its intended function adequately, it requires one 2×2 switch for each wavelength as well as four separate multiplexers. Further, the conventional OADM does not provide channel power balancing or overall power control. Additional components must be either incorporated into or interfaced to the conventional OADM 400 to provide these latter functions so as to prevent signal distortions which would otherwise arise from non-uniform power levels of signals propagating through optical amplifiers present within an optical communications network.
FIGS. 5A–5b illustrate two known OADM architectures based upon micro-mirror arrays. The OADM 500 (FIG. 5A) comprises a micro-mirror array 501a that comprises only one set of micro-mirrors 503.1–503.4 to facilitate both channel adding and dropping operations simultaneously; the OADM 550 (FIG. 5B) comprises a different micro-mirror array 501b that comprises a first set of mirrors 505.1–505.4 to facilitate channel dropping operations and a second set of mirrors 507.1–507.4 to facilitate channel adding operations. The micro-mirrors may be fabricated using either Micro-ElectroMechanical (MEMS) or micro-fluidic techniques, both of which are known in the art. Each of the mirrors 503.1–503.4, 505.1–505.4, 507.1–507.4 may assume one of only two positions or states—an “on” position whereby the mirror is disposed within the path of light comprising a channel so as to deflect the light and an “off” position whereby the channel light does not encounter the mirror. In FIGS. 5A–5b, mirrors in the “on” and “off” configurations are indicated by solid and dashed lines, respectively.
In both the OADM 500 (FIG. 5A) and the OADM 550 (FIG. 5B), a composite optical signal enters the respective OADM from an input fiber optic line is de-multiplexed into its component channels λ1, λ2, . . . , λn by de-multiplexer 502a. Simultaneously, a set of channels to be added are input to the respective OADM from add line 514 and are de-multiplexed into their component channels by de-multiplexer 502c. Each of the de-multiplexed channels leaving de-multiplexer 502a is collimated by a respective collimator lens 504 and is input to the adjacent micro-mirror array—array 501a in OADM 500 and array 501b in OADM 550.
Each of the mirrors comprising the OADM 500 (FIG. 5A) and the OADM 550 (FIG. 5B), in its “on” position, deflects the path of one channel to be dropped and/or one channel to be added. The paths of one dropped channel λ2 and of one added channel λ′2 are illustrated, respectively, by solid and dashed lines in FIGS. 5A–5b. In the OADM 500 (FIG. 5A), both of the channels λ1 and λ′1 (if present) will encounter and be deflected by the mirror 503.1 if this mirror is in its “on” configuration. Likewise, both of the channels λ2 and λ′2 (if present) will encounter and be deflected by the mirror 503.2, both of the channels λ3 and λ′3 (if present) will encounter and be deflected by the mirror 503.3 and both of the channels λ4 and λ′4 (if present) will encounter and be deflected by the mirror 503.4, if the respective mirror, in each case is “on”. Any mirror in an “on” position will cause one signal from the input line to be dropped to the drop line and/or will cause another signal from the add line to be added to the output line, wherein the added and dropped channels have the same wavelengths. In the OADM 550 (FIG. 5B), each pair of mirrors 505.1 and 507.1, 505.2 and 507.2, 505.3 and 507.3 and 505.4 and 507.4 functions in a coordinated fashion such that either both of or neither of the mirrors comprising each pair are in their “on” states. As shown in FIG. 5B, the deflection caused by one mirror of each pair causes one signal to be dropped while the deflection caused by the other mirror causes a signal of the same wavelength to be added.
The collimated light of each channel exiting the apparatus 500 or the apparatus 550 to either the drop line or to the output line is collected by one of the focusing lenses 506 from which it is directed to either the multiplexer 502d or the multiplexer 502b. Each multiplexer combines the various channels which it receives from the micro-mirror array 501a–501b into a single composite optical signal.
Although the micro-mirror based OADM's 500 and 550 utilize an elegant architecture and appear to perform their intended functions adequately, they suffer the drawback that the free-space path length through each apparatus increases proportionally to the total number of channels. The diameter of the collimated light of each channel increases as this free-space path length increases, thereby requiring high performance levels for the collimator lenses and tight tolerances for the mirror positions. This leads to difficulties in achieving and maintaining alignment between the various collimator and focusing lenses associated with each wavelength. Further, neither of these micro-mirror based OADM's provides channel power balancing or overall power control. Additional components must be either incorporated into or interfaced to the conventional OADM 500 or the OADM 550 to provide these latter functions.
Glossary
In this document, the individual information-carrying lights are referred to as either “signals” or “channels.” The totality of multiple combined signals in a wavelength-division multiplexed optical fiber, optical line or optical system, wherein each signal is of a different wavelength range, is herein referred to as a “composite optical signal.”
The term “wavelength,” denoted by the Greek letter λ (lambda) is used herein synonymously with the terms “signal” or “channel.” Although each information-carrying channel actually comprises light of a certain range of physical wavelengths, for simplicity, a single channel is referred to as a single wavelength, λ, and a plurality of n such channels are referred to as “n wavelengths” denoted λ1–λn. Used in this sense, the term “wavelength” may be understood to refer to “the channel nominally comprised of light of a range of physical wavelengths centered at the particular physical wavelength, λ.”
Strictly speaking, a multiplexer is an apparatus which combines separate channels into a single wavelength division multiplexed composite optical signal and a de-multiplexer is an apparatus that separates a composite optical signal into its component channels. However, since many multiplexers and de-multiplexers ordinarily operate in either sense, the single term “multiplexer” is usually utilized to described either type of apparatus. Although this liberal usage of the term “multiplexer” is generally used in this document, the exact operation—either as a multiplexer or a de-multiplexer—of any particular apparatus should be clear from its respective discussion.
According to the above discussion, there is a need for an integrated optical component which can simultaneously perform optical demultiplexing, adding and dropping of multiple channels, power balancing among channels, and control of overall optical power levels. The present invention addresses such a need.